1. Field of the Invention
This invention relates to a vehicle wheel alignment apparatus which comprises eight sensors for measuring the toe angles of the wheels of a vehicle. More particularly, the invention relates to an apparatus which comprises means for computing the toe angles of the wheels using only six of the sensors in the event that any of the sensors becomes inoperative, and means for comparing the toe angles computed using separate sets of sensors to determine if the apparatus requires calibration.
2. Description of Related Art
Vehicle wheel aligners comprising a toe angle measurement system having eight sensors, or angle measuring instruments, are known in the art. For example, U.S. Pat. No. 4,383,370 issued to Van Blerk et al., which is owned by the assignee hereof, discloses a toe angle measurement system comprising eight electro-mechanical sensors. U.S. Pat. No. 5,220,399 issued to Christian et al., which is also owned by the assignee hereof, discloses a toe angle measurement system comprising eight optical sensors, including optical sensors for measuring the angles between the planes of diagonally-opposed wheels. These and other existing eight-sensor toe angle measurement systems provide increased accuracy when determining the toe angles of the rear wheels of a vehicle.
One problem with many vehicle wheel aligners is that, if a toe angle sensor becomes inoperative, for example if a service technician inadvertently blocks the light beam being transmitted between cooperating optical sensors, the toe angle measurement system is interrupted and toe angle information will consequently not be displayed or if the information is displayed, it will not be current. Another problem with vehicle wheel aligners is that the toe angle measurement system may become out of calibration. If an out of calibration condition is not detected by the service technician, erroneous alignments will result.